A polymer of a vinyl ether is one of useful monomers since the polymer is used as an adhesive, a coating material, a lubricant, an elastomer, grease, and the like. In particular, a polyvinyl ether having a hydroxyl group on the side chain thereof, has characteristics including hydrophilicity, adhesiveness to a substrate, crosslinking reactivity, and the like, and is expected to be applied to various purposes. Furthermore, the polyvinyl ether having a hydroxyl group on the side chain thereof greatly changes in the hydrophilicity and hydrophobicity thereof, and thus is also useful as a temperature responsive polymer.
A copolymer of a vinyl ether and another polymerizable monomer can have another property of the additional polymer imparted thereto in addition to the properties of the vinyl ether, and in particular, a block copolymer thereof can be utilized as a polymer surfactant, a thermoplastic elastomer, a coating material, an adhesive, a template agent for lithography, and the like. However, since a vinyl ether generally has an electron donating substituent and is cationically polymerizable, it is difficult to provide a copolymer with a radically polymerizable monomer, and various investigations have been made for providing the copolymer.
For example, as a method for providing a block copolymer of a cationically polymerizable vinyl ether and a radically polymerizable monomer, such a method has been proposed that living cation polymerization of a vinyl ether is performed with a vinyl ether having a group capable of undergoing reversible addition fragmentation chain transfer polymerization as an initiating species, and living radical polymerization of a radically polymerizable vinyl monomer is performed with the resulting polyvinyl ether as a macro chain transfer agent (PTL 1). However, with a vinyl ether having a hydroxyl group, the hydroxyl group inhibits cation polymerization, and thus a block copolymer cannot be obtained by the method.
A vinyl ether having a hydroxyl group can be reduced in electron donating property by using an aqueous solvent, such as an alcohol, through hydrogen bond between the aqueous solvent and the hydroxyl group, and thereby a polymer can be obtained by radical polymerization (PTL 2). However, the radical polymerizability of the vinyl ether having a hydroxyl group is lower than the other radically polymerizable monomers, and in this method, it is difficult to provide a copolymer of a vinyl ether having a hydroxyl group and another radically polymerizable monomer, particularly a copolymer having a molecular weight, a molecular weight distribution, and a compositional ratio that are controlled.
As a method for providing a polymer having a molecular weight, a molecular weight distribution, and a composition that are controlled with a radically polymerizable monomer, the living radical polymerization methods, such as atom transfer radical polymerization (ATRP) and reversible addition fragmentation chain transfer polymerization (RAPT polymerization), have been widely known. In particular, RAFT polymerization is featured by the use of a polymerization initiator and a reversible addition fragmentation chain transfer agent (RAFT agent) and has relatively high versatility as compared to other living radical polymerization, in such points as (1) the applicability to various monomers and (2) the wide range of application in reaction condition.
The RAFT agent used may be a thiocarbonylthio compound (R-SC(S)Z) containing a moiety (R) forming a radical through addition fragmentation and a moiety (Z) adjacent to the thiocarbonyl group, and for imparting livingness to radical polymerization, it is necessary to select the optimum RAFT agent depending on the reactivity of the monomer. For general vinyl monomers having radical polymerizability, the selection standards have been established for a wide range of monomers, and for example, it has been known that a high active RAFT agent, such as a dithioester and a trithiocarbonate, is suitable for a conjugated monomer, such as a methacrylate ester, styrene, an acrylate ester, acrylic acid, acrylamide, and acrylonitrile, and a low active RAFT agent, such as a dithiocarbamate and a xanthate, is suitable for an unconjugated monomer, such as vinyl acetate, N-vinylpyrrolidone, and an N-carbazole (NPLs 1 and 2). However, there has been no example of RAFT polymerization considered for a vinyl ether having a hydroxyl group, and there has been no knowledge about the polymerizability thereof in the presence of a RAFT agent, a suitable RAFT agent, and the like.